Fretello - Duolingo for learning how to play guitar
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Uberchord Guitar - Get better on your guitar. Fast.
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Justin Guitar - Over 750 completely free guitar lessons!
soundrop - Soundrop is an app that allows you to create your own personalized sound track to keep you focused or relax.